1. Field of the Invention
The present invention relates to a semiconductor device and, more particularly, to a semiconductor device for use in an optical pick-up of an optical data recording and reproducing apparatus.
2. Description of the Prior Art
Recent years have seen a variety of optical heads (also known as optical pick-ups) developed for use in optical data recording and reproducing apparatuses. These optical pick-ups are illustratively used to detect pit information from compact discs (CD's).
FIGS. 1 and 2 are views of the optical pick-up and its optical integrated circuit which this applicant proposed in Japanese Patent Application No. 1-99221. In FIG. 1, an optical pick-up setup 1 comprises an optical integrated circuit 2, an objective lens 3 and an optical disc 4. In FIG. 2, the optical integrated circuit 2 comprises a semiconductor device 9 mounted on the principal plane of a semiconductor (silicon) substrate 5. The semiconductor device 9 contains a first and a second photodiode 6 and 7 as well as a transistor circuit 8. The transistor circuit 8 is located at the periphery of the photodiodes 6 and 7 and constitutes a differential amplifier or its equivalent receiving signals from these photodiodes. On the surface of the semiconductor substrate 5 containing the photodiodes 6 and 7 as well as the transistor circuit 8, there is fixedly deposited a prism 10 having a semitransparent reflecting surface 10a at an angle of 45 degrees (=.theta.). Opposite to the semitransparent reflecting surface 10a is an LOP (laser on photodiode) chip with a semiconductor laser 13 mounted on a silicon chip 12 containing a monitoring photodiode 11. When the optical pick-up 1 is in operation, a light beam 14 from the semiconductor laser 13 is reflected on the semitransparent reflecting surface 10a of the prism 10. The reflected light beam passes through the objective lens 3 and is focused onto the optical disc 4. After being reflected by the optical disc 4, the returning optical beam again passes through the objective lens 3, is refracted on the semitransparent reflecting surface 10a, and enters the prism 10. The light beam is detected first by the first photodiode 6. The light beam is then reflected, and is detected by the second photodiode 7. The monitoring photodiode 11 detects the optical intensity of the laser beam emitted by the semiconductor laser 13.
The first and second photodiodes 6 and 7 for optical signal detection are each made of a three-part diode construction. The diodes pick up reproduced signals, and detect focusing and tracking errors.
In the semiconductor device 9 containing the photodiodes 6 and 7, reflection control films (also known as optical films) are formed on the diode surfaces. The reflection control films are provided in such a manner that the amount of the light incident on the photodiode 6 via the prism 10 becomes substantially the same as the amount of light incident on the photodiode 7 via the same prism. The reflection control film is provided generally by utilizing (i) a field oxide film (SiO.sub.2); (ii) a inter-layer insulation film in the case of multilayer wiring; (iii) an overcoat (passivation) layer; (iv) a double layer film combining any two of the three layers (i) through (iii); or (v) an oxide film constituting the MIS capacity. That is, no extra production stages are added in manufacturing the reflection control film.
The incident light entering the prism 10 is directed only to the photodiodes 6 and 7 and is kept from the transistor circuit 8 by a light-shielding aluminum layer formed thereon.
As described; the semiconductor device 9 has its reflection control films made of the above materials (i) through (v) (especially on the first photodiode 6). One disadvantage of these reflection control films is that they do not fully function as a full-fledged reflection control film structure. Their original purposes require these films to be uneven in thickness and make it difficult for them to attain the level of film thickness precision needed to implement adequate reflection control. If the emphasis were placed on the reflection control function of these films, their original functions as the field oxide film, inter-layer insulation film and/or overcoat layer would have to be sacrificed at least in part. This dilemma may not be apparent when photodiodes are provided as discrete devices. The trouble becomes evident when photodiodes are formed in an IC format (the so-called single-chip format), as in the case of the photodiodes 6 and 7 as well as the transistor circuit 8 being mounted on the same semiconductor substrate 9.
On the other hand, the light-shielding aluminum layer is characterized by its relatively large area. This can cause the so-called stress migration in the aluminum layer when that layer is covered with the overcoat layer, the stress migration possibly leading to the disappearance of the aluminum layer.
One solution to the layer disappearance mentioned above is to selectively remove only the overcoat layer from the light-shielding aluminum layer. This can worsen the adhesive property of the layer relative to the prism 10 and can result in a reduced level of the operational reliability.